Invasive Species Report

A member of the Asteraceae family, Italian thistle is an annual herb native to the Mediterranean region and is widespread in California, Oregon and Washington, however it is not found east of the Sierra Nevada. It was accidentally introduced into United States (Batra et al. 1981) and California (Goeden 1974) in the 1930s. Robbins (1940) reports it as early as 1912 near Fort Bragg in Mendocino County. It forms a deep taproot and prefers fertile, well drained soils but is found in disturbed areas, roadsides, pastures, meadows and grasslands. It dominates sites and crowds out native species and discourages wildlife from entering infested areas. It grows well in oak savanna and can carry grass fires to tree canopies. Although Italian thistle can grow to over six feet it is usually knee high and is often present in clusters. Its leaves are white-woolly below, hairless-green above and deeply cut into two to five pairs of spiny lobes. Stems are slightly winged. The thimble-sized flower heads in pastel shades of rose, pink to purple flowers are clustered in groups of two to five are covered with densely matted, cobwebby hairs. Italian thistle is bisexual and a single plant can produce 20,000 seeds in one season (Wheatley and Collett 1981). Its light seeds are spread by lodging (bent or broken stems in contact with the ground), wind, vehicles, and animals and also may spread from seed-contaminated hay and soil from infested quarries. To remove Italian thistle dig them out 2-4 inches below the soil before flowering. Mowing is a waste of time, in fact, plants cut 4 days after flowering can still produce viable seed. Italian thistle seedbank may last up to 10 years. Intensive grazing by sheep and goats is effective. A pre-emergent and growth regulator such as Milestone is one of the most effective herbicides for thistles and generally does not harm grass. Did I say don’t touch Italian thistle? Wow does it hurt! Use your thickest gloves!

-Mark Skinner: Invasive Species Chair